Alexei Kniazev

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PNe in Nearby Galaxies

M31

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New Planetary Nebulae in the Outer Disk and Halo of M31:

(SDSS projects in collaboration with Grebel, E.K., Zucker, D.B., Bell, E.F., Rix, H.-W., Holtzman, J., D. Martinez-Delgado)

Search for PNe candidates in SDSS M31 data.

We have developed a method to select PNe candidates in Sloan Digital Sky Survey (SDSS) imaging data based on photometric criteria. Using an SDSS scan of M31, we recovered a number of previously-known PNe from Nolthenius & Ford (1987,ApJ,317,62; NF87 hereafter) and Jacoby & Ford (1986,ApJ,304,490; JF86), and developed selection criteria on the basis of their colors and magnitudes in the SDSS data. We then applied these criteria to the whole area of M31 as scanned by SDSS, selecting ~90 new PN candidates and 30 known PNe. We estimated possible contamination by point sources such as QSOs and stars, analyzing their distribution from Richards et al. (2002,AJ,123,2945). We also estimated possible contamination by background emission-line galaxies, using data from Kniazev et al. (2004,ApJS,153,429). The numbers of all these types of objects in our sample should be extremely small, since PNe stand out as being bluer in (g-r) vs. (r-i), and redder in (u-g) vs. (g-r), due to the contribution of the very strong emission lines Hbeta and [OIII] 4959,5007 in the SDSS g-band and Halpha in the SDSS r-band.

Color-magnitude diagrams for stellar sources from SDSS M31 data to which we applied our selection procedure. All previously known re-identified PNe from the literature are shown with red filled circles. Their locus defines our "first priority" candidates. Our color-magnitude criteria for selecting these candidates are shown with green lines. Blue lines in the bottom-right (u-g) vs. r diagram show softer criteria for selection of the "second priority" candidates. All identified PNe are located within a relatively small range of luminosities of ~1.5 mag that shows the depth of previously published surveys for PNe in the outer disk and halo of M31. Since the detected flux from PNe comes almost entirely from nebular emission lines in the optical, the range of colors characteristic of the detected PNe is defined by the ratios of these emission lines and their corresponding contributions in different SDSS passbands.

The spatial distribution of all selected PNe candidates from M31 SDSS data.The figure coordinates X and Y are in arcdegrees from the center of M31, along the major and minor axes, respectively. With blue triangles all stellar PNe candidates of the first priority are shown. With red filled circles all stellar PNe candidates of the first priority that were identified from the earliest works are shown. With pink triangles all stellar PNe candidates of the second priority are shown. With yellow diamonds all extended PNe candidates of the second priority are shown. With green diamonds all extended PNe candidates of the first priority are shown.

Follow-up spectroscopy.

Spectroscopic observations of potential PNe were obtained with CAFOS on the 2.2m telescope at Calar Alto Observatory (Spain), on the nights of 2004 October 7--14. In 8 nights of observations under variable weather conditions (seeing varied from 0.8 to 2.5 arcsec, a total of 86 candidates were observed. 74 first and second priority ``stellar'' PNe candidates (i.e., those identified as point sources by the SDSS pipeline) were observed, in addition to 8 first priority spatially-extended candidates. Only four second priority extended candidates were observed -- those lying at the greatest distances from the center of M31 -- and we do not include them in our efficiency statistics. In the end, 70 PN candidates appeared to be genuine PNe, yielding a total efficiency of $\sim$85% for our method. Among stellar first and second priority targets, the efficiencies were 89% and 79%, respectively.

Top-left: Spectrum of one newly discovered PN, showing the strongest emission lines, Hbeta, [O III] 4959,5007 and Halpha. Top-right: Color-magnitude diagram for stellar sources from SDSS M31 data to which we applied our selection procedure. Our color-magnitude criteria for selecting PNe candidates are shown with green lines. Blue lines show less stringent criteria for the selection of "second priority" candidates. All true PNe confirmed with spectroscopic follow-up observations are shown with red filled circles; observed PN candidates without obvious emission lines are shown with blue filled circles. Bottom Panels: Final PN distributions in g and r bands. Blue histograms indicate the distribution of observed PN candidates without obvious emission lines. The data have been binned into 0.2 mag intervals.

The spatial distribution of PNe from the M31 SDSS data. Crosses are PNe found in this study, filled red circles are those from NF87 and JF86 also recovered using our criteria. PNe from NF87/JF86 that were reobserved with our spectroscopy are marked with both symbols. The black ellipse indicates the extent and inclination of M31's disc, and large black circles mark the locations of NGC205, M32 and Andromeda NE. The blue ellipse shows the main area of the disk of M31 surveyed for PNe with the Planetary Nebula Spectrograph (Merrett et al.,2003,MNRAS,346,62L). We assume a distance to M31 of 760 Kpc.

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South African Astronomical Observatory

Last Updated: 10.December.2005